hey there
i have a diesel surf and it cost 60 to fill from red approx, out of that i can get aroud the 400 or 500 kilometres out of it approx 300 mile, depending on driving style and speed and also town or motorways.
different factors can be the serviced state of your engine, the tyres you use, the pressure in the tyres, the quality of the fuel, and a whole host of other things
i have noticed a small difference after using redex, i put it in about once a month, as directed, some reccomend that if you get down to red and chuck a bottle in and blast it down the motorway that will clean otu the engine also....
hope this helps
graham